As a method for removing volatile materials in a polymer, there are proposed, for example, in Japanese Patent Publication Nos. 29245/1986 and 52163/1986, methods wherein a molten thermoplastic resin is processed in a vent extruder, or in Japanese Patent Application (OPI) No. 166506/1984, a method wherein a polymeric liquid composition is continuously devolatilized by a volatilization separator directly connected to a vertical bubbling-type preheater and a vacuum tank. In recent years, a high-pressure extraction method that uses a supercritical fluid has been proposed, and Japanese Patent Publication No. 46972/1984 discloses, as a method for reducing oligomeric cyclic ethers contained in tetrahydrofuran polymers, alkylene oxide polymers, or tetrahydrofuran/alkylene oxide copolymers, a method wherein such a polymer is brought in contact with a gas that is in the supercritical state.
However, in the methods described in the above Japanese Patent Publication Nos. 29245/1986 and 52163/1986 and Japanese Patent Application (OPI) No. 166506/1984, the reduction in the concentration of volatile materials remaining in the processed polymer is approximately at the order of, at the most, 400 ppm, and these methods cannot meet the market demand where the concentration of volatile materials is required to be lower than 400 ppm. Further, although the method described in Japanese Patent Publication No. 46972/1984 is a method for extracting impurities from a polymer in the liquid state at ordinary temperatures under high pressure, thereby removing the impurities, the method cannot be applied to high-degree extraction wherein the concentration of impurities remaining in a polymer is required to be reduced to or below a certain concentration (thousands ppm).